1. Field of the Invention
The present invention relates to a method for producing golf balls, and more particularly to a method for producing golf balls having a coating film which is in good adhesion to the golf ball.
2. Description of the Prior Art
Golf balls including thread-wound golf balls and solid golf balls such as one- or two-piece solid golf balls are always provided with a surface coating. The surface coating plays an important role not only in the protection of letters, numerals, and symbols stamped on the golf ball, and of surface gloss and good appearance, but also in the reduction of air resistance which leads to improvement in their flying performance.
The surface coating is usually formed from a polyurethane paint. However, it will peel off upon intense impact by a golf club if it is formed after molding by mere application of a polyurethane paint to the surface of a golf ball. For the improvement of adhesion between a golf ball and a surface coating, several attempts have been made to perform surface treatment on the surface of the golf ball. None of them have achieved satisfactory results and there remain many problems unsolved in the manufacturing process.
There is a known method for surface coating which consists of treating the surface of a golf ball with flame prior to coating. This method is not desirable because it is in danger of fire and burn. Moreover, it does not provide-sufficient film adhesion. There is another known method for surface coating which consists of roughening the surface of a golf ball by sandblasting or the like. This method does not improve the film adhesion as desired, but it seriously damages the dimples formed in the surface.
There is disclosed in Japanese Patent Publication No. 39266/1988 a method for improving film adhesion to golf balls by surface treatment with a low-pressure plasma. A disadvantage of this method is the necessity of a vacuum chamber for plasma treatment which has to be isolated from the atmosphere by a valve, and continuous treatment depends greatly on the life of the valve. Another disadvantage is that the plasma treatment under low-pressure is not useful for golf balls covered with balata resin which gives off volatile ingredients under reduced pressure.
There is disclosed in Japanese Patent Laid-Open Nos. 15171/1990, 48626/19901 241739/1991 and 236475/1991 a method for treating the surface of an object with a glow plasma obtained under atmospheric pressure. This method is suitable for flat objects but is not suitable for spherical objects like golf balls.